STEM Readiness: Biology Readiness - Molecular Biology: DNA Replication - Semi-conservative, Key Enzymes, Leading/Lagging Strand

Must?Know (20–25 detailed bullets)

• Prokaryotic cells range from 0.1–5.0 ?m in diameter; eukaryotic cells range from 10–100 ?m.
• DNA in prokaryotes is located in the nucleoid, an unenclosed region; eukaryotes house DNA within a membrane-bound nucleus.
• Prokaryotes have 70S ribosomes (30S + 50S subunits); eukaryotes have 80S ribosomes (40S + 60S subunits).
• Mitochondria and chloroplasts in eukaryotic cells contain 70S ribosomes, supporting their origin via endosymbiosis.
• Bacterial cell walls contain peptidoglycan; archaea lack peptidoglycan and have pseudopeptidoglycan or other polymers.
• Plant cell walls are composed of cellulose; fungal cell walls contain chitin; animal cells lack a cell wall.
• All cells have a phospholipid bilayer plasma membrane; only eukaryotes have internal membrane systems (e.g., ER, Golgi).
• Eukaryotes possess membrane-bound organelles: nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes.
• Prokaryotes lack membrane-bound organelles; metabolic functions occur in the cytoplasm or at the plasma membrane.
• Bacteria (e.g., Escherichia coli) and archaea (e.g., Methanogens) are prokaryotes; animals, plants, fungi, and protists are eukaryotes.
• Mycoplasma species are bacteria that lack a cell wall, making them resistant to penicillin.
• Human red blood cells lack a nucleus and mitochondria, maximizing space for hemoglobin.
• Plant cells contain chloroplasts for photosynthesis; animal cells do not.
• Lysosomes are membrane-bound organelles in animal cells containing hydrolytic enzymes; plants use vacuoles for similar degradation.
• The nuclear envelope is a double membrane with nuclear pores that regulate transport between nucleus and cytoplasm.
• Chromosomes in prokaryotes are typically a single, circular DNA molecule; eukaryotes have multiple linear chromosomes.
• Plasmids are small, circular DNA molecules found in prokaryotes and some yeasts (eukaryotes), often carrying antibiotic resistance.
• Endosymbiotic theory proposes that mitochondria and chloroplasts originated from engulfed prokaryotes.
• Evidence for endosymbiosis includes: 70S ribosomes, circular DNA, and binary fission in mitochondria and chloroplasts.
• Prokaryotes reproduce by binary fission; eukaryotes divide by mitosis and cytokinesis.
• Flagella in prokaryotes are made of flagellin and rotate; eukaryotic flagella are composed of microtubules (9+2 arrangement) and bend.
• Cilia are short, numerous motile structures in some eukaryotes (e.g., Paramecium); prokaryotes lack cilia.
• The nucleolus within the nucleus is the site of ribosomal RNA synthesis and ribosome assembly.
• Peroxisomes contain enzymes like catalase that break down hydrogen peroxide; present in both plant and animal eukaryotic cells.
• Glyoxysomes are specialized peroxisomes in plant seeds that convert fats to sugars.

Difficulty Level

Intermediate – expected foundational knowledge in first-semester biology but requires precise differentiation of structures and exceptions.

Common Traps (3–5 factual traps)

• Trap: All cells with cell walls have peptidoglycan – Fact: Only bacteria have peptidoglycan; plants (cellulose), fungi (chitin), and archaea (other polymers) have different wall compositions.
• Trap: Ribosome size correlates with organism complexity – Fact: Mitochondria and chloroplasts have 70S ribosomes despite being in eukaryotic cells.
• Trap: Prokaryotes have no DNA packaging – Fact: Prokaryotic DNA is supercoiled and associated with nucleoid-associated proteins, though not histones.
• Trap: Eukaryotes are always multicellular – Fact: Many eukaryotes (e.g., yeast, Amoeba, Paramecium) are unicellular.

Practice MCQs (5–7 questions)

Question: Which of the following is a feature found in prokaryotic cells but not in eukaryotic cells?
A) Circular DNA
B) 70S ribosomes
C) Nucleoid region
D) Plasma membrane
Answer: C
Explanation: The nucleoid is an unenclosed DNA region unique to prokaryotes.
Why the top distractor is wrong: 70S ribosomes are in prokaryotes but also in mitochondria and chloroplasts of eukaryotes.

Question: A cell is observed to contain mitochondria, lysosomes, and a nucleus but lacks a cell wall. This cell is most likely from:
A) A bacterium
B) A fungus
C) A plant
D) An animal
Answer: D
Explanation: Animal cells have membrane-bound organelles and no cell wall.
Why the top distractor is wrong: Fungi have lysosomes and a nucleus but possess a chitin-based cell wall.

Question: Which structure is evidence supporting the endosymbiotic theory?
A) Presence of a nucleus
B) 80S ribosomes in the cytoplasm
C) Circular DNA in mitochondria
D) Linear chromosomes in the nucleus
Answer: C
Explanation: Mitochondria have circular DNA, similar to bacteria, supporting their prokaryotic origin.
Why the top distractor is wrong: 80S ribosomes are standard in eukaryotic cytoplasm and not evidence of endosymbiosis.

Question: Which of the following organisms lacks a cell wall?
A) Saccharomyces cerevisiae
B) Escherichia coli
C) Mycoplasma pneumoniae
D) Arabidopsis thaliana
Answer: C
Explanation: Mycoplasma is a bacterium that naturally lacks a cell wall.
Why the top distractor is wrong: Saccharomyces cerevisiae (yeast) is a fungus with a chitin-containing cell wall.

Question: What is the primary component of the cell wall in plants?
A) Peptidoglycan
B) Chitin
C) Cellulose
D) Lipopolysaccharide
Answer: C
Explanation: Plant cell walls are primarily composed of cellulose.
Why the top distractor is wrong: Peptidoglycan is found in bacterial cell walls, not plants.

Question: Which of the following is true about ribosomes in eukaryotic cells?
A) All ribosomes are 80S, including those in mitochondria
B) Ribosomes are only found free in the cytoplasm
C) 80S ribosomes are found in the cytoplasm; 70S in mitochondria
D) Ribosomes are enclosed within the nucleus
Answer: C
Explanation: Cytosolic ribosomes are 80S; mitochondrial ribosomes are 70S.
Why the top distractor is wrong: Mitochondrial ribosomes are 70S, not 80S.

Question: Which cellular structure is present in both prokaryotes and eukaryotes?
A) Nucleus
B) Endoplasmic reticulum
C) Plasma membrane
D) Mitochondria
Answer: C
Explanation: The phospholipid bilayer plasma membrane is universal to all cells.
Why the top distractor is wrong: The nucleus is exclusive to eukaryotes.

Last?Minute Revision (20–25 one?liners)

• Prokaryotic cell size: 0.1–5.0 ?m; eukaryotic: 10–100 ?m.
• Prokaryotes have nucleoid; eukaryotes have nucleus with double membrane.
• Prokaryotic ribosome = 70S; eukaryotic cytoplasmic ribosome = 80S.
• Mitochondria and chloroplasts have 70S ribosomes and circular DNA – evidence of endosymbiosis.
• Bacteria: peptidoglycan cell wall; archaea: no peptidoglycan.
• Plants: cellulose cell wall; fungi: chitin; animals: no cell wall.
• All cells have phospholipid bilayer; only eukaryotes have internal membranes.
• Eukaryotic organelles: nucleus, mitochondria, ER, Golgi, lysosomes, peroxisomes – absent in prokaryotes.
• Mycoplasma – smallest known bacterium, lacks cell wall.
• Human red blood cells lack nucleus and mitochondria.
• Binary fission in prokaryotes vs. mitosis in eukaryotes.
• Plasmids – small circular DNA, common in bacteria, can carry antibiotic resistance.
• Nuclear pores regulate RNA and protein transport across nuclear envelope.
• Nucleolus = site of rRNA synthesis and ribosome subunit assembly.
• Lysosomes in animal cells; vacuoles perform degradation in plants.
• Chloroplasts only in plants and some protists; absent in animals.
• Prokaryotic flagella: flagellin, rotary motion; eukaryotic: microtubules (9+2), bending motion.
• Cilia only in eukaryotes (e.g., Paramecium).
• Endosymbiotic theory explains origin of mitochondria and chloroplasts.
• Mitochondria divide by binary fission, similar to bacteria.
• Glyoxysomes in plant seeds convert fats to carbohydrates.
• Peroxisomes contain catalase to break down H?O?.
• Archaea are prokaryotes but differ from bacteria in membrane lipids and cell wall chemistry.
• Histones are associated with eukaryotic DNA; most prokaryotes lack histones (some archaea have histone-like proteins).
• Verify from standard textbook: Exact protein composition of archaeal cell walls.
• Size difference: eukaryotic cells are typically 10x larger in diameter than prokaryotes.